// Copyright (c) 2015-2019 Amanieu d'Antras and DGuco(杜国超).All rights reserved.
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.

namespace stagefuture
{
namespace detail
{

// Thread-safe singleton wrapper class
#ifdef HAVE_THREAD_SAFE_STATIC
// C++11 guarantees thread safety for static initialization
template<typename T>
class singleton
{
public:
    static T &get_instance()
    {
        static T instance;
        return instance;
    }
};
#else
// Some compilers don't support thread-safe static initialization, so emulate it
template<typename T>
class singleton {
    std::mutex lock;
    std::atomic<bool> init_flag;
    typename std::aligned_storage<sizeof(T), std::alignment_of<T>::value>::type storage;

    static singleton instance;

    // Use a destructor instead of atexit() because the latter does not work
    // properly when the singleton is in a library that is unloaded.
    ~singleton()
    {
        if (init_flag.load(std::memory_order_acquire))
            reinterpret_cast<T*>(&storage)->~T();
    }

public:
    static T& get_instance()
    {
        T* ptr = reinterpret_cast<T*>(&instance.storage);
        if (!instance.init_flag.load(std::memory_order_acquire)) {
            std::lock_guard<std::mutex> locked(instance.lock);
            if (!instance.init_flag.load(std::memory_order_relaxed)) {
                new(ptr) T;
                instance.init_flag.store(true, std::memory_order_release);
            }
        }
        return *ptr;
    }
};

template<typename T> singleton<T> singleton<T>::instance;
#endif

} // namespace detail
} // namespace stagefuture
